The loudspeaker as an energy conversion device exhibits its own motion characteristics under excitation. Its various modes of resonance at different frequencies depends on a multitude of mechanical and electrical design parameters. It remains a designer's dream to have flat magnitude-frequency and linear phase-frequency characteristics.
A common technique for modifying the magnitude-frequency characteristic of the input electric signal and thus modifying the magnitude-frequency of the acoustic output is to filter the input in a selective manner. A band of pink noise 1/3 octave wide is fed into the loudspeaker for sound pressure measurement at a fixed distance from the loudspeaker. Signal gain in this particular band can then be changed accordingly. Obviously, this conventional method of "equalizing" is a very coarse adjustment--only the averaged deviation can be corrected. Two undesirable side effects occur--overlap in adjacent band pass filters and phase irregularities at the band edges.
Ishii et al. (U.S. Pat. No. 4,015,089) disclosed a multi-driver speaker system wherein the the relative positions of the drivers along the radiation path helps to create a cancellation of sound waves at a particular frequency. This cancellation results in a favorable condition for a smooth phase characteristic when a particular crossover network is used. The claim to flat amplitude and linear phase response seems groundless in a strict sense.
Berkovitz et al. (U.S. Pat. No. 4,458,362) uses an adaptive filter to equalize signals for room acoustic compensation. In the same patent it was shown that the same adaptive process can be used for loudspeaker performance improvement. While the adaptive process is desirable for room acoustic compensation, it does not represent what can be achieved ultimately for loudspeaker sound improvement. Though the advantage of the Widrow-Hoff adaptation algorithm is that prior knowledge of the speaker characteristic is not needed, the algorithm generates only approximate values for filter coefficients through stochastic approximation. In terms of loudspeaker sound improvement, an one-time operation, more accurate results can be obtained by the deteministic process of the current disclosure instead of stochastic approximation.
Serikawa et al. (U.S. Pat. No. 4,751,739) corrects the speaker sound pressure frequency characteristic by multi-band digital filters with desired frequency reponses. The coefficients of these filters are generated by inverse Fourier transform of a transfer function resulting from repeated Hilbert transforms and modifications. However, while the Hilbert transforms render the resultant time sequence causal, phase linearity is lost.